A wireless communication system using a protocol called “CSMA/CA (carrier sense multiple access/collision avoidance) system has been known as a unit for realizing a wireless communication between vehicles without using a base station. In the CSMA/CA system, each communication apparatus confirms that a communication channel is continuously free for a given time or longer, postpones data transmission if there is no free communication channel, and thereafter transmits data, thereby avoiding a collision between data transmitted from each communication apparatus.
Also, as disposed in Patent Document 1, the application of the CSMA/CA system for an inter-vehicle communication has been known. In particular, a technique has been known in which information is shared between the vehicles by conducting a one-way broadcast communication without ACK (acknowledgement) as the inter-vehicle communication of the CSMA/CA system.
Patent Document 1: JP-A-2009-278536
In the inter-vehicle communication, because each vehicle travels, it is important to ensure a real-time property of the data communication.
However, in the inter-vehicle communication, because each vehicle periodically starts transmission at an arbitrary timing, it is conceivable that transmission is frequently postponed if the number of peripheral vehicles increases. For that reason, the inter-vehicle communication in which information on vehicle states are transmitted and received among the respective vehicles suffers from such a problem that a detection of a change in a state of another vehicle is delayed.